Facet fixation prosthesis

ABSTRACT

A facet screw system for surgical implantation into bone tissue having a shaft, a compression member and a washer. The shaft includes a shaft having a bone engaging portion and a compression member engaging portion. The compression member includes a spherical head portion having a recess for engaging with a driving instrument, and an elongated coupling portion having internal threads for coupling with the compression member engaging portion of the shaft and a washer coupled to the spherical head portion of the compression member and having a plurality of bone engaging protrusions. The washer is adapted to be polyaxially rotatable with respect to the compression member.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S.application Ser. No. 12/359,706 filed on Jan. 26, 2009, which is anon-provisional application that claims priority to provisionalapplication 61/023,197 filed on Jan. 24, 2008, all of which areincorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The present invention relates to a device to stabilize and support thespine.

CROSS-REFERENCE TO RELATED APPLICATIONS

All publications and patent applications mentioned in this specificationare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

BACKGROUND OF THE INVENTION

Degenerative disc disease refers to any kind of degradation of theintervertebral disc. Patients suffering from degenerative disc diseasewith severe or chronic back pain or instability are commonly treadedwith spinal fusion. Fusion of the lumbar spine typically involvescombining a bone graft/implant with fixation devices to add extrastabilization of the spine while the area heals, increasing the chanceof a successful fusion.

Traditionally this fixation is provided by pedicle screws and rods inorder to support and stabilize the spine until boney fusion occurs.Pedicle screw fixation has been the gold standard for such stabilizationsince the late 1980s. However, pedicle screw fixation is invasive andhas been shown to injure the paraspinous muscles causing scarring andother complications. Recently facet fixation has grown in popularity asa less invasive means of providing the posterior stabilization necessaryin lumbar fusion cases. There is a need in the spine industry for afast, simple, and minimally invasive means to instrument an interbodyfusion involving facet fixation.

SUMMARY OF THE INVENTION

A facet screw system for surgical implantation into bone tissue having ashaft, a compression member and a washer. The shaft includes a shafthaving a bone engaging portion and a compression member engagingportion. The compression member includes a spherical head portion havinga recess for engaging with a driving instrument, and an elongatedcoupling portion having internal threads for coupling with thecompression member engaging portion of the shaft and a washer coupled tothe spherical head portion of the compression member and having aplurality of bone engaging protrusions. The washer is adapted to bepolyaxially rotatable with respect to the compression member.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more readily understood with reference to theembodiments thereof illustrated in the attached drawing figures, inwhich:

FIG. 1 is a perspective view of a prosthetic device in accordance withan embodiment of the invention;

FIG. 2 illustrates the components of the prosthetic device in accordancewith an exemplary embodiment of the invention;

FIG. 3 illustrates the shaft portion of the prosthetic device inaccordance with an exemplary embodiment of the invention;

FIG. 4 shows the cross-sectional view of the shaft portion of theprosthetic device in accordance with an exemplary embodiment of theinvention;

FIGS. 5 and 6 illustrate different views of the compression member inaccordance with an exemplary embodiment of the invention;

FIGS. 7 and 8 illustrate different views of the washer in accordancewith an exemplary embodiment of the invention;

FIG. 9 illustrates the cross-sectional view of the connection betweenthe shaft, the compression member and the washer in accordance with anexemplary embodiment of the invention;

FIGS. 10 and 11 illustrate the prosthetic device positioned in the spinein accordance with an exemplary embodiment of the invention;

FIG. 12 illustrates another embodiment of the prosthetic deviceaccording to the present invention;

FIG. 13 is another view of the prosthetic device according to thepresent invention;

FIG. 14 shows another view of the components of the prosthetic deviceaccording to the present invention;

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will now be described. The followingdetailed description of the invention is not intended to be illustrativeof all embodiments. In describing embodiments of the present invention,specific terminology is employed for the sake of clarity. However, theinvention is not intended to be limited to the specific terminology soselected. It is to be understood that each specific element includes alltechnical equivalents that operate in a similar manner to accomplish asimilar purpose.

With reference to FIGS. 1 and 2, one embodiment of a facet screw system10 according to the present invention is illustrated. The facet screwsystem 10 includes a screw shaft 12, a compression member 14, and awasher 16. The facet screw 10 may be constructed from any biocompatiblematerial including, but not limited to, stainless steel alloys,titanium, titanium based alloys, or polymeric materials.

Referring to FIGS. 2 and 3, the compression member 14 has a sphericaltop portion that is engageable with the spherical curvature of thewasher 16. It is also desirable to prevent the compression member fromcompletely passing through the washer 16. This may be achieved by makingthe diameter of the bottom portion of the washer 16 opening, throughwhich the threaded portion of the shaft may pass, as least between 5%and 10% smaller than the diameter of the spherical top portion of thecompression member 14. To permit rotation or swiveling o the shaftrelative to the washer 16, the diameter spherical top portion of thecompression member may be from about 5% to about 30% less than thediameter of the washer 16 opening.

The shaft 12 which engages with compression member 14 may also include arecess 18 for receiving a driving instrument. As is well known in theart, the recess 18 may be configured and dimensioned to be any shapethat corresponds with the end of the driving instrument designed toengage the screw shaft portion 12.

The shaft portion 12 of the facet screw 10 comprises a shaft having alength L surrounded at least in part by a plurality of thread portions20 a, 22 a. The shaft 12 having a length L may vary depending upon theanatomy of the spine. The following lengths are provided in a preferableembodiment of the present invention.

Bone Engaging Compression Assembled Length with Portion MemberCompression Member   10 mm 15 mm 30 mm 12.5 mm 15 mm 40 mm   15 mm 15 mm50 mm   25 mm 15 mm 60 mm

The diameter of the bone engaging portion 20 is preferably 5.0 mm to 6.0mm and the compression member engaging portion 22 has a diameter in therange of 4.4 mm to 4.9 mm. In a preferred embodiment, the diameter ofthe compression member engaging portion 22 remains generally constantfrom the proximal end toward the distal end of the shaft 12. However,the diameter of the bone engaging portion 20 preferably decreasestowards the distal tip 24 of the facet screw 10. The constant diameterof a majority portion of the bone engaging portion 20 allows for optimalscrew positioning when the bone screw is inserted into a predeterminedarea in the bone tissue. In another embodiment, the diameter of theshaft 12 may vary along its length, including increasing in diameterfrom the proximal end to the distal end or decreasing in diameter fromthe proximal end to the distal end.

Looking at FIGS. 3 and 4, the plurality of threads surrounding the shaft12 extend, in a preferred embodiment, from the distal tip 24 of theshaft 12 to the distal end 26 of the shaft 12. In the preferredembodiment, the threads of the bone engaging portion 20 of the shaft 12are dual lead thread type and the tube engaging portion 22 are providedwith a STD machine thread. However, any type of thread for eitherportion 20, 22 may be used to facilitate the function of the facet screw10.

In another preferred embodiment, facet screw 10 may also include atleast one flute to clear any chips, dust, or debris generated when thefacet screw 10 is implanted into bone tissue. In yet another preferredembodiment, the facet screw 10 may be cannulated so that a guide wiremay be used for positioning the facet screw into the bony elements ofthe spine.

Now turning to FIGS. 5 and 6, the compression member 14 according to thepreferred embodiment is illustrated. The compression member 14preferably has standard female threads that mate with the male threadson the compression member portion 22 of the shaft portion 12 of thefacet screw 10. In the preferred embodiment, the compression member 14is configured and dimensioned with a spherical head 28 having a recessfor receiving a driving instrument. The outer diameter of thecompression member 14 is preferably the same as the outer diameter ofthe bone engaging portion 20 of the shaft 12 of the facet screw 10.

When the compression member 14 is rotated clockwise, the compressionmember 14 advances distally, thereby shortening the length between thebone engaging surface 20 of the shaft portion 12 and the washer 16 ofthe facet screw 10. During this step, the facet screw 10 is held inplace while only the compression member 14 is rotated. As the washer 16is advanced to contact the facet joint, compression of the facets occursproviding greater stability and fixation of the superior and inferiorfacets.

FIGS. 7 and 8 illustrate the washer 16 in one embodiment of the presentinvention. The washer 16 is preferably pre-assembled as a single piecehaving a flexible portion 30 and bone engaging portion 32. The washer 16has an opening having a generally spherical curvature which the shaftportion of the facet screw is retained within. It is desirable toprevent the shaft 12 from passing completely through the washer 16.Therefore, a ridge 37 is provided on the inner circumference of theflexible portion 30 of the washer 16.

The flexible portion 30 is provided with a plurality of slits 34 toallow the washer 16 to be flexible or resilient for engaging with thespherical head 28 of the compression member 14. The bone engagingportion 32 is configured and dimensioned with a plurality of protrusions36 to engage with bone. The spherical head 28 of the compression member14 and the flexible portion 30 of the washer 16 are configured toangulate with respect to one another. The washer 16 is configured toangulate with respect to the compression member 14, thereby enabling thewasher 16 to conform to various angle differences of the superior facetwhile the facet screw 10 is advanced a different trajectory across thefacet joint.

The bone engaging portion 32 of the washer 16 is configured anddimensioned so that any load may be distributed evenly when the shaftportion of the facet screw is inserted at a non-orthogonal angle. In oneembodiment, the bottom surface of the washer 16 lies substantially flushwith the surface of the bone. In another embodiment, the bone engagingportion 32 is configured with a plurality of protrusions to promote bonecontact over a substantial portion of the bottom portion of the boneengaging portion 32 of the washer 16 even when the bony surfacecontacting the crown is uneven or has a depression in it. In addition,the protrusions 36 may help minimize the potential for screw orcompression member back-out.

In another embodiment, each of the plurality of protrusions 36 whichengage the bone may be configured to achieve maximum amount of purchasein the bone by providing protrusions that more readily permits slidingin one direction while more substantially resisting motion in a seconddirection. Thus, angled or curved protrusions may be used and aplurality of them may be oriented on the bottom surface of the crown topermit rotational sliding one direction more readily than in anotherdirection. One advantage of an angled or curved protrusion is that itcan be driven into the bone by applying a given amount of torque.Another advantage of an angled or curved protrusion is that it may beconfigured and dimensioned such that it provides more resistance tomotion in one direction than in the other direction.

In some applications, it may be desirable to allow a surgeon to changethe geometry of some or all portions of the washer 16 based on the shapeof the bone. This provides the advantage of allowing a surgeon tovisually inspect the bone and configure the shape of the washer 16accordingly. It may be desirable to change the shape so that there isincreased contact between the bone engaging portion of the washer 16 andthe surface of the bone. This is because the load can only bedistributed over the areas of washer 16 that in contact with the surfaceof the bone. An uneven contact with the bone may cause the load to beincreased over the areas of the washer 16 that are in contact with thebone. This is undesirable for many reasons, for example, because it doesnot all the washer 16 to distribute the load in a predictable manner.This may have many detrimental effects including, but not limited to,cracking, chipping, or breaking of the surface of the bone.

FIG. 9 shows the cross-sectional view of the facet screw system inaccordance with an exemplary embodiment of the invention. Asillustrated, the washer 16 is polyaxially rotatable with respect to thespherical head of the compression member 14. The washer 16 may beprovided with a ridge 37 which creates an interference area with thespherical head of the compression member 14. The washer 16 adapted toangulate between 0 to 25 degrees with respect to the compression member14 and the shaft 12. Also illustrated in FIG. 9, is an undercut 19 thatis provided on an upper portion of the shaft 12 so that a removal toolmay be used if necessary. The removal tool is capable of engaging withthe undercut 19 and configured to remove the shaft 12 from the bonetissue.

In many applications, it is desirable to prevent back out of the screw.One way to do this is by creating an interference area, as describedabove. In one embodiment, a single continuous ridge or a plurality ofridges may be selectively positioned along the inner surface of thewasher 16. One advantage of the ridge 37 is that they createinterference between the washer 16 and the screw shaft 12 such that thewasher 16 would have to flex out for the screw to back out, or to beunscrewed. This can be accomplished by using a removal tool whichengages with the undercut 19 and unscrews the shaft 12 from the bonetissue.

FIGS. 10 and 11 illustrate the facet screw system 10 positioned in thespine according to the preferred embodiment. In the preferred embodimentof the present invention the facet screw 10 is positioned in the bonetissue through the inferior facet 38 and the superior facet 39. As thefacet screw 10 is advanced into the bone the inferior and superiorfacets are compressed together. As a result, the motion between thefacets is prohibited thereby enhancing fusion of the spine segment. Itis generally preferably to position two facet screw 10 on opposing facetjoints on a single level of the vertebrae.

FIG. 12-14 illustrates another embodiment of the facet screw assembly 40of the present invention. In this particular embodiment, the facet screwassembly 40 is provided with a shaft 42, a washer 44, and a nut 46. Theshaft 42 is comprised of a nut engaging portion 48 and the bone engagingportion 50. The shaft is also provided with a bone graft window portion52 that is positioned between the nut engaging portion 48 and the boneengaging portion 50

The nut engaging portion 48 of the shaft 42 is provided with a pluralityof teeth 54 that engages with the nut 46. The nut 46 is configured withat least one teeth engaging protrusion 56. The teeth engaging protrusion56 is configured so that once the nut 46 can be advanced towards thedistal end of the shaft 42, the protrusion 56 engages with the teeth 54and prohibits the backing out of the nut 46 from the shaft 42. Asmentioned in more detail above, the washer 44 is spherical dimensionedand configured so that it may angulate with respect to the nut 46. Thewasher 44 may also be provided with a plurality of protrusions to engagebone tissue. The plurality of protrusions may be optimally configured tostabilize the facet screw with bone tissue.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations can be made thereto by those skilled in the art withoutdeparting from the scope of the invention as set forth in the claims.

What is claimed is:
 1. A facet screw system for surgical implantationinto bone tissue, comprising: a shaft, the shaft having a bone engagingportion, a graft window, and a compression member engaging portion; acompression member, the compression member comprising: a spherical headportion having a recess for engaging with a driving instrument, and acoupling portion for coupling with the compression member engagingportion of the shaft; and a washer coupled to the spherical head portionof the compression member, wherein a diameter of the spherical headportion of the compression member is greater than a diameter of a bottomportion of the washer.
 2. The facet screw system of claim 1, wherein thefacet screw system is cannulated for receiving a guide wire.
 3. Thefacet screw system of claim 1, wherein a flute is provided at a distalend of the shaft.
 4. The facet screw system of claim 1, wherein thewasher comprises a plurality of slits extending from a bottom portion ofthe washer through the top portion of the washer.
 5. The facet screwsystem of claim 1, wherein the spherical head of the compression memberis configured to receive the driving instrument.
 6. The facet screwsystem of claim 1, wherein the shaft portion comprises a recess forreceiving the driving instrument.
 7. The bone screw of claim 1, whereinthe shaft portion is configured with one type of thread and thecompression member engaging portion is configured with another type ofthread.
 8. The bone screw of claim 1, wherein the screw is made fromstainless steel alloys, titanium, titanium based alloys, or polymericmaterials.
 9. A facet screw system for surgical implantation into bonetissue, comprising: a shaft, the shaft having a bone engaging portion, agraft window, and a compression member engaging portion; a compressionmember having a proximal end and a distal end, the compression membercomprising: a spherical head portion located at the proximal end andhaving a recess for engaging with a driving instrument; and an elongatedcoupling portion having internal threads for coupling with thecompression member engaging portion of the shaft; and a washer having aridge positioned on a top portion of the washer and plurality of boneengaging protrusions on a bottom portion, wherein the washer ispolyaxially rotatable with respect to the compression member.
 10. Thefacet screw system of claim 9, wherein the facet screw system iscannulated for receiving a guide wire.
 11. The facet screw system ofclaim 9, wherein a flute is provided at a distal end of the shaft. 12.The facet screw system of claim 9, wherein the spherical head of thecompression member is configured to receive the driving instrument. 13.The facet screw system of claim 9, wherein the shaft portion comprises arecess for receiving the driving instrument.
 14. The bone screw of claim9, wherein the shaft portion is configured with one type of thread andthe compression member engaging portion is configured with another typeof thread.
 15. The bone screw of claim 9, wherein the screw is made fromstainless steel alloys, titanium, titanium based alloys, or polymericmaterials.
 16. A facet screw system for surgical implantation into bonetissue, comprising: a shaft, the shaft having a bone engaging portion, agraft window, and a compression member engaging portion; a compressionmember having a proximal end and a distal end, the compression membercomprising: a spherical head portion located at the proximal end andhaving a recess for engaging with a driving instrument; and an elongatedcoupling portion having internal threads for coupling with thecompression member engaging portion of the shaft; and a washer having aplurality of bone engaging protrusions on a bottom portion, the washerbeing coupled to the spherical head portion of the compression member,wherein the washer comprises a plurality of slits extending along a topportion of the washer, wherein the slits extend to an edge of the topportion of the washer and enable engagement with the compression member.17. The bone screw of claim 16, wherein the slits do not extend throughthe bottom portion.
 18. The bone screw of claim 16, wherein an outerdiameter of the elongated coupling portion of the compression member issubstantially the same as an outer diameter of the bone engaging portionof the shaft.
 19. The bone screw of claim 16, wherein the washerincludes an opening extending through the top portion and the boneengaging portion, and a diameter of the opening being greater in thebone engaging portion relative to the top portion.
 20. The bone screw ofclaim 16, wherein the washer extends around an entire periphery of thecompression member.